1. Field of the Invention
The present invention is related to a method and apparatus for three dimensional (3D) surface measurement, and more particularly related to a method and apparatus for analyzing absolute contour of full fielded 3D surface of objects applying a projected moire fringe interferometry.
2. Description of the Prior Art
Measuring contour of three dimensional surface of objects attracts attention in the society of engineering and technology. The currently available commercial apparatus for 3D surface measurement includes a type of instrument for measuring three-coordinate data of objects which is relied on mechanically contacting each point of the object surface, and another type of laser instrument of measuring three-coordinate data of objects. The instrument for the contacting measurement is equipped with a mechanical probe. The probe which is driven by a numerical control mechanism travels on the surface of an object under measurement to thereby provide the three-coordinate data for every point of the object surface. Accuracy of the spatial measurement is generally better than 0.01 mm for the instrument. However, a measuring speed of the instrument is relatively slow, since it takes times for the numerical system to move across the surface while the mechanical probe applies forces at certain extent to the object.
In stead of applying the mechanical probe, the laser instrument employs an optical probe which is driven by a numerical control system. An optical spot generated by the optical probe scans surface of an object. In general, the laser instrument improves the measuring speed while suffering a loss of some degree of the accuracy in the spatial measurement. However, increase of the measuring speed for the laser instrument is still limited by the moving speed of the numerical control system. Therefore, a full field measurement of the object will be optimum if it can largely increase the measuring speed.
A moire image including moire fringes is obtained from applying a projected moire interferometry. The fringes are produced from interfering an optical image of a master grating, when the image is optically positioned on a submaster grating. The projected moire interferometry is a full field non-contacting technique for measurement of characteristics of the object surface, which possesses a plurality of measuring capabilities as compared with a holographic interferometry. However, it is more important that, the projected moire interferometry enables controlling its sensitivity in measurement so that the moire interferometry is excellent to reject external interference. For this reason, there is a great prospect of utilizing the projected moire interferometry in engineering applications.
A moire contouring is a promising optical method for imaging object surface, which is originally introduced by Takasaki and Meadows et. al. An apparatus which applies the method is actually quite simple, including a grating which is positioned adjacent an object so that shadow fringes can be observed on the object after projecting lights through the grating. The shadow fringes are equivalent to lines of the moire contouring under certain conditions, which can be used to measure surface characteristics of objects. The method is particularly useful to measure objects having small sizes since the grating size limits application of the apparatus only to the small objects.
Another method of the moire contouring is respectively introduced by Benoit, P.; Yoshino, Y.; Suzuki, M. and others. The method includes projecting an image of grooves of a master grating onto the surface of an object to thereby image a graph of the moire contour of the object through a submaster grating. The method is referred to the projected moire interferometry, which is particularly useful to measure objects having large sizes. Under certain conditions, beat fringes generated from a combining effect of the master grating and submaster grating provide lines of a contour map of the imaged object surface, which is analogous to the way a topographic map delineates the contour of the land. In the early 1980s, image processing was successfully introduced for analyzing the moire fringes. Core technologies of the image processing particularly include a phase shift algorithm and unwrapping algorithm, which makes the projected moire fringe interferometry enable to perform a real-time measurement.
According to studies of Meadows, Takasak and Suzuki et. al, the moire fringes become the respective lines of the surface contour map for an object if following criteria are met: optical centers of the respective projection and imagining optical axe are in parallel; spaces of the respective projecting master grating and imaging submaster grating are the same; focal lengths of the respective projection lens and imaging lens are the same; and distance between the projection grating and lens is the same as compared with distance between the imaging grating and lens.
Referring to FIG. 1, there is illustrated a prior art moire interferometer, wherein a light source 1 projects light rays which pass through a projection master grating 2, so that an image of grooves of the master grating is focused on an object 4 under measurement after the lights optically pass through a projection lens 3. An image of the measured object is then positioned on a submaster grating 6 in addition to moire fringes which are also formed. Images of the formed moire fringes are then optically recorded by a camera 8 through a camera lens 7.
However, there are two unresolved problems which have persistently associated with the projected moire interferometry long time ago. The first one is that contour lines of the contour map for the object surface, which are described by the moire fringes, are function of orders of the fringes. Therefore, differences in attitude are not equal for respective two adjacent contour lines of the map. In fact, the attitude difference is also a function of the fringe order. Therefore, it is necessary to accurately determine the absolute orders of the moire fringes if absolute appearance of the 3D object surface is desired. The second problem is a necessity of accurately measuring object distances between the respective projection device to the object, and the imaging device to the object, and imaging distances between the respective projection grating to the object, and the image grating to the object. The currently available projected moire interferometer is unable to accurately measure the above mentioned distances. In stead, it is from a rough measurement to estimate the object distances and image distances, according to an assumption that the attitude differences between the respective two adjacent contour lines are constant.